1. Field of the Invention
The present invention relates generally to antennas; and more particularly, to dual polarized antennas.
2. Description of the Prior Art
In general, dipole antennas have been used for a long time, and many variations have been developed over the years. "Bow tie" dipoles operate like any ordinary half-wavelength dipole, and are described in several textbooks, including Balanis, Constantine A., "Antenna Theory Analysis And Design", Wiley, 1997.
With the increasing popularity of polarization diversity techniques in mobile communications, dual polarized antennas have become more important. These are antennas that radiate two orthogonal polarizations, such as vertical/horizontal (0.degree. & 90.degree.) or +/-45.degree. slant polarizations. Many types of dual polarized antennas have been investigated and are widely available on the open market.
These antennas are divided into two groups:
1. Antennas that utilize single linear polarized elements, but are grouped and fed in such a manner to create a dual polarized array. An example is a patch array (or dipole array), where two separate patches (or two separate dipoles), are required to radiate both polarizations.
2. Antennas that utilize dual polarized elements to make a dual polarized array. Examples are a single patch that radiates two different polarizations, or two crossed dipoles that are constructed in such a manner as to become a single dual polarized element.
Feeding techniques are also a competitive area. Many vendors use coaxial cable, or Teflon dielectric microstrip transmission lines. Antennas that use coaxial cable or Teflon microstrip transmission lines will suffer from reduced efficiency, and possibly generate third-order intermodulation distortion.
Antennas that utilize single linear polarized elements need to have them carefully placed on the ground plane (reflector) in order to radiate symmetrical patterns. Also, good port-to-port isolation (between the two inputs) can be very difficult to achieve on an antenna that has a reflector crowded with many elements. When using air dielectric transmission lines, the process of feeding the radiators can also become very unwieldy with so many varying locations for signals to be fed.
Dual polarized antennas that utilize dual polarized elements suffer from other problems. Crossed dipole elements need to be extra-long to provide good intra-element (within the same dual-polarization element isolation), this leads to a dipole impedance which is so high (200 ohms) as to make it difficult to match over a broad bandwidth. Even without an extra-long element, the dipole impedance is high (150 ohms).
A single dual polarized patch antenna has poor port-to-port isolation, bandwidth, and cross-polarization discrimination characteristics; while many of these problems can be minimized with various techniques, the trade-off analysis is a delicate process.
Propagating radio waves are weakened and distorted by the environment in which they travel. In addition, when two waves arrive at the same point with an opposite phase and equal amplitude, they cancel one another out, resulting in a phenomenon known as multipath fading. Many cellular phone connections are typically lost due to multipath fading. One solution known in the art to this problem is a spatial diversity technique, wherein two different antennas are used and separated, for example, by about 20 wavelengths, for receiving (or transmitting) the same information on two separate radio signal paths. However, one problem with such an approach is that two antennas are needed to receive (or send) one signal, while communities are trying to minimize the number of antennas.
In view of the above, there is a real need in the prior art for an antenna that solves the multipath fading problem, that reduces the number of antennas, that solves the coaxial cable dielectric signal loss problem, that eliminates unnecessary solder joints, screw connections and pressure connections, and that is easily manufactured.
Moreover, a very important aspect of a dual polarization antenna is isolation between the two different inputs that correspond to the two different polarizations. Isolation in this case is defined as a ratio of power leaving one port to the power entering the other port. Ideally the ratio of power will equal 0.0 in terms of linear magnitude or -.infin. dB (decibels), which means that all power entering a port will be radiated by the antenna, or reflected back through the same port, which is represented by a non-ideal voltage standing wave ratio (VSWR). But realistically a ratio of 1/1000 to 1/100 (or -30 to -20 dB) is an attainable goal for isolation. A good isolation characteristic is important to a user, especially when used in a configuration where the antenna is used for transmission and for reception. This is because some of the transmitted power, if the isolation characteristic is bad, will leak back into the other port and overwhelm the receiver attached thereto.
Degradation in isolation can arise from several sources such as: (1) Leakage in radio frequency (RF) energy from the feed system of one polarization to the feed system of the opposite polarization; (2) Intra-element coupling, arising from RF energy "leaked" within a single dual-polarized element, from one dipole to its opposite polarized dipole, which then makes its way back to the opposite input port; and (3) Inter-element coupling that arises from RF energy which couples from one polarization to the opposite polarization, but only between adjacent (dual-polarized) elements, which then makes its way back to the opposite input port.
Techniques used in the past vary for non-bow tie cross dipole antennas, including careful arrangement of radiating elements on the reflector, careful selection of dipole length, the addition of such things as additional walls (or "fences") between radiating elements, or additional walls lengthwise in the array plane.
But these approaches and the cross dipole antennas resulting therefrom have some shortcomings. Careful arrangement of radiating elements on the reflector cannot be done in the case of dual polarized cross bow tie dipoles because this technique needs separate radiating elements, which can be moved relative to each other. Walls or fences between radiating elements may have a result of contributing to a cross polarization component in the far field radiation pattern. Walls or fences lengthwise in the array plane have a result of narrowing the azimuth beamwidth, and also contribute to a cross polarization component in the radiation pattern. These techniques have worked with plain cross dipoles in the past, however, they have not been shown to be effective with dual polarized antennas having cross bow tie dipoles.
The above mentioned devices do not contribute significantly toward improving isolation for cross bow tie dipole antennas. In view of the above, there is a real need in the art for an antenna that solves these problems.